[Thierry88]

It is not the bites or scratches of a walker that makes you turn, it just causes a fatal infection with a high fever like they are having a huge amount of nasty bacteria in their saliva. Then after you die, you turn, just because you died and not because you were bitten.

Remember Jim from the first season, he got bit but not in a fatal part. He got a severe fever and was ultimately left behind to die and turn afterwards. On the other hand, there was Amy, who was bit as well. However she did not die from the infection by the bite but because she lost a lot of blood due to her carotid artery being severed. And yet she turned, without going through a period of fever, just because she died.